Composition and Separation of Dry Air for the ESAT
Updated July 2026
Dry air is a stable mixture of gases, primarily nitrogen and oxygen, with smaller amounts of noble gases and carbon dioxide. Understanding its precise composition and the industrial process of fractional distillation is essential for the ESAT Chemistry section, as these gases are vital industrial feedstocks.
Dry air is a mixture consisting of approximately 78% Nitrogen (), 21% Oxygen (), 0.9% Argon (), and 0.04% Carbon Dioxide (). These components are separated industrially by liquefying the air and using fractional distillation to exploit differences in their boiling points.
The Composition of Dry Air
Air is a complex mixture of various gases. When chemists refer to the composition of dry air, they are describing the atmosphere with all water vapour removed. This is because the concentration of water vapour is highly variable, ranging from near 0% in arid regions to approximately 5% in tropical, humid environments. By considering dry air, we can define a consistent set of proportions for the remaining gases.
The four primary components of dry air that you must know for the ESAT are:
- Nitrogen (): Approximately 78%.
- Oxygen (): Approximately 21%.
- Argon (): Approximately 0.9%.
- Carbon Dioxide (): Approximately 0.04%.
While other noble gases like neon, helium, and krypton are present, they exist only in trace amounts. It is important to note that air is a mixture, not a compound. The gases are not chemically bonded to one each other and, therefore, retain their individual physical properties, such as their specific boiling points. This characteristic allows them to be separated via physical means.
Separating the Components of Air
Because the components of air are gases with very different boiling points, they can be separated using fractional distillation. This industrial process involves cooling the air until it turns into a liquid (liquefaction) and then gradually warming it up so that each component evaporates at its specific boiling point.
Step 1: Liquefaction and Purification
Before air can be distilled, it must be liquefied. This is achieved through a cycle of compression and cooling:
- Filtration: The air is filtered to remove dust and other solid particulates.
- Removal of Water and Carbon Dioxide: The air is cooled to below 0 °C to remove water vapour (which freezes into ice) and further cooled to remove carbon dioxide (which turns into a solid at -78.5 °C). This step is critical because these solids would otherwise block the narrow pipes and heat exchangers of the distillation plant.
- Compression and Expansion: The remaining gases (mostly nitrogen, oxygen, and argon) are compressed to high pressures, which causes them to warm up. They are then cooled by heat exchangers and allowed to expand rapidly. This rapid expansion causes a significant drop in temperature. This cycle is repeated until the air reaches a temperature of approximately -200 °C, at which point it becomes a pale blue liquid.
Step 2: Fractional Distillation
The liquid air is pumped into the bottom of a fractionating column. As the liquid air enters the column, it is allowed to warm up slightly. The fractionating column is designed so that it is warmer at the bottom and cooler at the top.
As the temperature rises, the components of the liquid air reach their respective boiling points and boil off (vaporise) at different levels:
- Nitrogen has the lowest boiling point at approximately -196 °C. It boils first and rises to the top of the column, where it is collected as a gas.
- Argon has a boiling point of approximately -186 °C. It boils off after nitrogen and is collected from the middle sections of the column.
- Oxygen has the highest boiling point of the three at approximately -183 °C. It remains a liquid for longer and is collected from the bottom of the column.
By carefully controlling the temperature gradient in the column, high-purity samples of nitrogen, oxygen, and argon can be obtained for industrial and medical use.
Key takeaways
- Dry air is composed of 78% Nitrogen, 21% Oxygen, 0.9% Argon, and 0.04% Carbon Dioxide.
- Air is a mixture, meaning its components can be separated by physical processes like fractional distillation.
- Liquefaction requires cooling air to approximately -200 °C through repeated compression and expansion cycles.
- Nitrogen is the most volatile component and is collected at the top of the fractionating column due to its low boiling point of -196 °C.
Memorise the order of the boiling points. Nitrogen (-196 °C) is 'colder' (more negative) than Oxygen (-183 °C), so Nitrogen always boils off first. A common trick in the ESAT is to ask which gas remains liquid the longest.
Be careful with the term 'dry air'. If a question asks about the composition of 'air' without the word 'dry', the percentages might not sum to 100% because water vapour is present. However, for calculation purposes, always use the dry air values unless stated otherwise.
The separation of air is a physical change, not a chemical one. This is a fundamental concept: the intermolecular forces between the different molecules in the mixture are overcome by thermal energy, but the covalent bonds within the or molecules remain intact.
Frequently asked questions
Why is it necessary to remove carbon dioxide and water before distillation?
Water and carbon dioxide freeze into solids at temperatures much higher than the liquefaction point of nitrogen and oxygen. If they were not removed, they would create ice and 'dry ice' blockages in the machinery, potentially causing high-pressure failures.
Which gas is collected at the bottom of the fractionating column?
Oxygen is collected at the bottom because it has the highest boiling point (-183 °C) of the three main gases, meaning it stays in liquid form while the others evaporate.
Does the percentage of carbon dioxide in the air ever change?
While we use 0.04% as the standard value for dry air, the actual concentration has been rising due to human activity, such as the burning of fossil fuels, and it can vary slightly depending on local vegetation and industrial activity.